原特提斯洋在滇西南地区的俯冲记录:来自保山地块奥陶纪花岗岩的年代学、地球化学以及Hf-Nd同位素的支持

Subduction records of the Proto-Tethys Ocean in SW Yunnan: Supported by geochronology, geochemistry and Hf-Nd isotopes of the Ordovician granites in the Baoshan Block

中国西南地区在早古生代期间位于冈瓦纳大陆北缘,该地区保存有早古生代岩浆岩记录,它们的形成与原特提斯洋的演化密切相关。本文报道了滇西南地区保山地块平达乡附近的早古生代花岗岩的岩石学、全岩主-微量地球化学、LA-ICP-MS锆石U-Pb年龄、锆石Hf同位素和磷灰石Nd同位素组成,以探讨保山地块早古生代花岗岩的岩石成因、岩浆源区性质以及岩体形成的地球动力学机制。通过对保山地块平达花岗岩中岩浆锆石开展LA-ICP-MS U-Pb同位素测试,结果显示花岗岩形成于465.9~454.9Ma,是中-晚奥陶世岩浆作用的产物。平达岩体主体岩性为二长花岗岩,主要矿物组成为斜长石、石英和碱性长石(条纹长石和正长石),次要矿物为黑云母和白云母。本文报道的花岗岩样品显示出高的SiO_(2)(72.16%~76.87%)和Al_(2)O_(3)(12.50%~14.36%)含量,低的MgO(0.12%~0.64%)、Mg#(20.9~35.9)、Cr和Ni含量以及较高的Rb/Sr和Rb/Ba比值;结合花岗岩较高的A/CNK值(1.06~1.45),显示弱过铝质至强过铝质的特征,表明其属于S型花岗岩。此外,平达岩体二长花岗岩样品具有低的Al_(2)O_(3)/TiO_(2)比值和变化的CaO/Na_(2)O、Rb/Sr、Rb/Ba比值,表明其岩浆源区包含泥质岩石和砂质岩石,显示混合源区的特征。结合滇西南早古生代二长花岗岩变化较大的锆石εHf(t)值(-11.10~-4.05)、磷灰石εNd(t)值(-8.76~-6.96)和Hf同位素二阶段模式年龄(2140~1704Ma),表明它们的岩浆源区为古元古代地壳中泥质岩石和砂质岩石混合源区。综合滇西南地区广泛发育的早古生代岩浆岩以及同时期的蛇绿混杂岩记录,本文认为保山地块早古生代S型花岗岩是地壳沉积组分部分熔融的产物,形成于原特提斯洋西向俯冲相关的活动大陆边缘背景。

The Southwest China was tonically situated in the northern margin of Gondwana continent during the Early Paleozoic, thus the Early Paleozoic magmatic records in this area were closely related to geodynamic evolution of the Proto-Tethys Ocean. In this paper, we present petrography, whole-rock major- and trace-element, LA-ICP-MS zircon U-Pb ages, zircon in situ Hf and apatite Nd isotopic compositions for the Early Paleozoic granites in the Pingda area of Baoshan Block in Southwest Yunnan to decipher their petrogenesis, magma source, and geodynamic mechanism. The LA-ICP-MS U-Pb isotopic analyses of zircons from the granites of Pingda pluton in Baoshan Block show that they were formed during 465.9~454.9Ma, representing the product of Middle-Late Ordovician magmatic activities. The Pingda pluton is dominantly composed of monzogranite, with main mineral compositions of plagioclase, quartz and alkaline feldspar (perthite and orthoclase), and minor biotite and muscovite. Geochemical analyses show that these monzogranite samples in the Pingda region have high SiO_(2) (72.16%~76.87%) and Al_(2)O_(3) (12.50%~14.36%) contents, low MgO (0.12%~0.64%), Mg# values (20.9~35.9), Cr and Ni contents, as well as high Rb/Sr and Rb/Ba ratios. Together with their relatively high A/CNK values (1.06~1.45), these monzogranites belong to the weakly peraluminous to strongly peraluminous series, indicating a S-type granite affinity. In addition, the monzogranite samples of Pingda pluton have low Al_(2)O_(3)/TiO_(2) ratios and varying CaO/Na_(2)O, Rb/Sr and Rb/Ba ratios, indicating that the studied granites represent partial melting products of heterogeneous sources with variable proportions of pelite and psammite rocks. Furthermore, these monzogranite samples have variable zircon εHf(t) values (-11.10~-4.05), apatite εNd(t) values (-8.76~-6.96), and heterogeneous two-stage model ages (2140~1704Ma), indicating that their magma source was dominant by Paleoproterozoic pelite and psammite rocks. Combined with widely distributed Early Paleozoic magmatic rocks and contemporaneous ophiolitic mélange records in this area, we argue that the Early Paleozoic S-type granites in the Baoshan Block were originated from partial melting of crustal sedimentary rocks, indicating an active continental margin setting related to westward subduction of the Proto-Tethys Ocean.